Typically wells are drilled into the earth's crust to desired subterranean location such as oil and/or gas bearing formations, or other formations of interest, through the application of rotary drilling techniques. In the rotary drilling of a well, a drilling fluid is circulated downwardly through the drill string and into the borehole through one or more ports located in the drill bit at the bottom of the drill string. The drilling fluid then moves upwardly through the annular space between the drill stream and the wall of the well to the surface. The drill cuttings formed at the bottom of the well by the action of the drill bit are entrained within the drilling fluid for circulation to the surface. The most widely used bits in rotary drilling operation are roller bits in which roller cones are mounted on the bottom of the bit so that as the bit is rotated, the cutting teeth of the cones roll along the bottom of the bore hole. The roller cones, usually three in number, although bits with more or less than three cones are also known, are rotatably mounted on spindles which project inwardly and downwardly from arm members depending from the shank portion of the drill bit.
In the rotary drilling operation, the rock or other earth material at the bottom of the bore hole is broken up by the cutting teeth of the roller cone, primarily through the compressive stresses resulting from the weight of the drill string on the drill bit. The bit itself is rotated at a relatively high speed resulting in rotation of the roller cones on their respective spindles under conditions imposing severe stresses and shock forces. In order to increase bit life, various techniques have been used to impart abrasion resistance in the sliding bearing surfaces which are normally encountered in the typical bit structure along with the rolling bearing surfaces such as are provided by roller bearings or ball bearing.
Roller bits commonly employ a spindle or shank structure upon which a rolling cutter is mounted and which is stepped down to terminate in a relatively small diameter projection commonly referred to as a "pilot pin." A hardened "thrust button" may be inserted in the roller cone in a thrust bearing relationship with the end of the pilot pin. Another procedure, as disclosed for example in U.S. Pat. No. 4,136,748 to Dickerhoff, is to employ a flat annular thrust washer which is in a conforming relationship with the spindle shoulder, or "thrust flange" surrounding the pilot pin. The thrust washer, along with a tapered roller bearing carries the thrust forces between the spindle and the roller cutter during the drilling operation. The bit disclosed in Dickerhoff also employs a small sleeve bearing in the cone into which the reduced pilot pin of the spindle is journaled to transfer radial loads. As also disclosed in Dickerhoff, the thrust washer and the sleeve bearing into which the pilot pin projects may be replaced by a single unitary bearing that is pressed in the small forward bore of the roller cutting and carries both radial and thrust loads. In the Dickerhoff structure, the tapered roller bearing configuration carries a substantial portion of the radial load along with the small sleeve bearing component, and also shares the thrust load with the flat annular thrust bearing component adjacent the pilot pin.
U.S. Pat. No. 2,104,819 to Schlumpf et al discloses a roller bit in which the rolling cutters are mounted on their respective spindles by means of ball bearings and sliding bearings. The outer spindles are stepped down in order to provide a forward projection of reduced diameter. In the Schlumpf bit, a stepped down sleeve which rotates relative to both the cutter cone and the spindle is used to provide both radial and thrust bearing surfaces. The bearing sleeve may be formed of carburized steel in order to increase the bit life.
It is also a conventional practice in the construction of roller bits to form part of the sliding bearing surfaces between the spindle and the roller cone of metal surfaces incorporating an anti-galling material. One technique for incorporating such anti-galling material is to provide alternate areas of steel and anti-galling material through the use of fenestrations, or slots, or circular or other shaped indentations such as disclosed in U.S. Pat. Nos. 3,235,316 to Whanger and 4,499,642 to Vezirian et al. Vezirian et al, for example, discloses the use of copper based alloys in a fenestrated ring bearing of a relatively hard metal formed of a ferrous based alloy or other metals such as nickel, chromium, cobalt, tungsten, titanium and metal carbides. In Whanger, anti-galling metals are incorporated into slotted or indented surfaces formed in the pilot pin, a roller cutter bushing receiving the pilot pin, the annular thrust flange surrounding the pilot pin, or in the enlarged rear portion of the spindle shaft as a substitute for roller bearings. In Whanger, the most effective antigalling materials appear to be an alloy of 85% silver and 15% manganese, although other anti-galling composites based upon lesser amounts of silver with metals such as copper, zinc and tin are also disclosed.
It is also known in the art to employ sliding bearing bushings in roller bits which are formed of a powder metallurgical composite providing a porous matrix in which an anti-galling material is infiltrated into the matrix. For example, U.S. Pat. No. 4,172,395 to Keller discloses a journal bearing bushing which is produced by sintering an alloy powder mixed with graphite at a temperature of 2050.degree. F. for 40 minutes. The sintering operation is carried out in situ resulting in bonding of the porous matrix to the base metal of the cutter forging. At the conclusion of the sintering procedure, the porous matrix material is infiltrated with an anti-galling material such as silver, lead or plastic materials.
U.S. Pat. No. 4,207,658 to Sorenson discloses a roller cutting bit in which journal and pilot bearing bushings and thrust buttons are formed of powder metallurgical composites infiltrated with an anti-galling material. In Sorenson, the powder metallurgical components are formed of a low carbon, nickel-steel alloy powder which is pressed and sintered and then infiltrated with an anti-galling material such as silver alloys or babbitt metals. A specific anti-galling material is 85% silver--15% manganese alloy.